Gasoline, xylene, methylbutyl ketone, pentane, and other volatile inflammable liquids as well as many volatile liquids which are not inflammable, are conventionally transported in a tank truck having a tank shell into which the liquid is charged or bottom loaded, through a large capacity bottom loading valve. The liquid is transferred from a storage tank at high rates in the range from 100 gallons per minute (gpm) to about 1500 gpm or more. When the tank is being charged, the liquid displaces a large quantity of vapor from within the tank, and it is necessary, particularly in many metropolitan areas, to collect and return the displaced vapor to the storage tank through an emission collection hood which is fitted over a vent in the top of the tank shell. The importance of the vent valve being open when the tank shell is being charged extends to any tank shell whether mobile or stationary.
Safety precautions, particularly in the event of an accidental rollover of the tank truck, require that nothing protrude above overturn rails provided on the tank. Some pertinent regulations are set forth in U.S. Pat. No. 3,854,623 and, in more detail, in the U.S. Dept. of Transportation Regulations, which are hereby incorporated by reference as if fully set forth herein. It is therefore especially important to construct a valve with a low exterior profile.
Moreover, as stated in U.S. Pat. No. 3,854,623, there are many tanks in service fitted with hoods having a high profile, or overturn rails of insufficient height to use presently available hoods. U.S. Pat. No. 3,854,623 was directed to cope with the problem of non-specification, easily damaged hoods, by providing a collapsible resilient hood of a plastic material. My invention is directed to a solution of a different problem namely, inadvertently rupturing the tank shell during charging, comprising replacing the vent valve in the hood, whether now-too-high or not, with a novel diaphragm vent valve. Even an existing now-too-high hood fitted with the diaphragm vent valve of this invention is acceptably safe, and it is unnecessary to modify now-too-low overturn rails, because the vent valve opens into the tank shell.
U.S. Pat. No. 3,752,355 utilizes an inward opening vent valve on a tank, for a different function which requires a different structure. U.S. Pat. No. 1,666,935 discloses a vent valve opening inwardly into a tubular base mounted on a tank shell; again, the vent valve functions differently from my valve and has a different structure.
Those skilled in the art will recognize that the tank shell of a tank truck is not primarily designed as a pressure vessel, and if the vent valve is inadvertently left closed while the tank is being charged, the tank will rupture, and the result can be a catastrophe. In present systems the liquid being charged into the tank hydraulically actuates the vent, or the vent is opened mechanically and usually manually, or the vent is opened with compressed air or by a buildup of vapor pressure in the tank. Irrespective of how the vent is opened, in a typical construction, the loading valve of a storage tank or the bottom loading valve of a tank truck is prevented from being inadvertently opened by a heavy spring in cooperation with a mechanical detent. The mechanical detent is first manually released before the bottom loading valve may be opened. Examples of this construction are seen in valves distributed by Parker, Shanangurs, Philadelphia Valve, Emco-Wheaton, and Allegheny Valve. None of the systems has a vent valve assembly-bottom loading valve detent interlock, and none guarantees that the tank truck cannot be charged unless the vent valve is open. Presently used charging systems have an interlock which provides for automatically opening the vent valve when liquid is charged through the bottom loading valve, but the tank shell can be charged even if the vent valve fails to open. More specifically no present system guarantees the bottom loading valve cannot be opened unless the vent valve is open.